Heat treating malleablized white iron to control growth



United States Patent HEAT TREATING MALLEABLEZED WHITE iiiQN This invention relates to malleablized white iron and the process of making such malleablized material. More particularly, this invention relates to such materials which are characterized by a very small growth during the malleablizin g process.

Conventional malleablizing practice for white iron of the black heart American malleable iron or ferritic iron variety has consisted largely of packing such white iron castings in pots with slag, sand, silicon gravel, pebbles or mill scale, and annealing in batch-type periodic furnaces. The packing material, which minimized decarburization and distortion of the castings, necessitated excessively long annealing cycles of from 5 to days because of the thermal lag between the work and the furnace temperature. For most castings where precise tolerances are not necessary, improvements have been made on the above pot annealing process, including the use of electrical resistance and radiant tube furnaces of the open type in which the castings were open to the furnace atmosphere, which later were adapted to use with non-decarburizing controlled atmospheres produced by the partial combustion of natural gas, butane, propane, coke oven gas, and the like.

As pointed out above, while the improved furnaces were entirely satisfactory for parts whose tolerance requirements were not close, it was found that when white iron chains or links, such as those used to fabricate power transmitting belts or chain drives and the like, were annealed in a neutral or reducing atmosphere, the pitch or dimensions of the chain or the growth of the castings were greater and beyond tolerances than when the identical chain was annealed by the conventional gravel pack or pot annealing process.

It will at once be apparent that it is economically much more advantageous to anneal by means of an open or unsealed annealing process in a gaseous atmosphere which takes about 20 to 60 hours as compared to 5 to 10 days for the gravel pack process. On the other hand, it would be economically prohibitive to redesign all chain or link patterns to adjust for the increased growth which is inherent in the usual open annealing process.

From the above, it will be apparent that there is a definite need for adapting the open annealing process so that it can produce castings of white iron of desired dimensions without the prohibitive cost of redesigning all casting patterns.

Briefly stated, the present invention comprises malleablized white iron castings having precise desired final dimensions which are developed by open annealing in a neutral to reducing atmosphere, having therein a sufiicient amount of water vapor to cause slight decarburization of the white iron being annealed, the amount of water vapor being balanced with the maximum acceptable depth of decarburization and with the reducing power of the particular atmosphere employed. In the annealing process, the castings are heated above the transformation temperature and held at this temperature until the cementite and alpha iron are converted to gamma iron and graphitic carbon. Then the castings are cooled slowly through the transformation range to convert the gamma iron to alpha iron and graphite.

Those features of the invention which are believed to be novel are set forth in the claims appended hereto. However, the invention will be better understood and other advantages and objects thereof will be better appreciated from a consideration of the following description:

Generally speaking, the white iron materials to which the present invention is directed contain by weight from about 2 to 3 percent carbon, preferably 2 to 2.65 percent carbon, 0.90 to 1.40 percent silicon, 0.25 to 0.55 manganese, 0.05 to 0.18 sulfur, and up to about 0.18 percent phosphorus with the remainder essentially iron.

As pointed out above, any of the usual non-decarburizing atmospheres may be used, such as exothermic gas or purified exothermic gas obtained by the partial combustion of natural gas, butane, propane, coke oven gas, and the like. For example, in using a gas produced from natural gas, an air-to-natural gas volumetric ratio is maintained, at about 9 /2 to 1, providing a purified exothermic atmosphere having volumetric analysis of 5.5% carbon monoxide, 5.5% hydrogen, and the balance essentially nitrogen. Other atmospheres include hydrogen, nitrogen, disassociated ammonia, and the like. The water content of the atmosphere is controlled by controlled humidification or dehumidification, mixing of wet and dry gases and other equivalent procedures. Preferably in connection with this invention, the dew point is maintained at from about 0 F. to about +50 F. Most preferably the dew point is maintained at from about +20 F. to +50 F., and the specific preferred dew point temperature is +45 F.

As pointed out above, the thermal cycle used in connection with the invention consists of heating the material or casting above the transformation temperature, holding at this temperature until the cementite and alpha iron are converted to gamma iron and graphitic carbon, and then cooling through the transformation range slowly to convert gamma iron to alpha iron and graphite. It has been found that the holding time above the transformation temperature can vary widely from about 2 to 60 hours, depending upon the temperature which broadly ranges from about 1550 F. to 1800 F., and preferably from about 1600" F. to 1750 F. Most preferably the castings are held at a temperature of 1700 F. plus or minus about 20 F. for about 20 hours. After holding above the transformation temperature for the requisite number of hours, the material is cooled as rapidly as the particular furnace will permit, to the second stage temperature range, which varies broadly from Il420 F. to

1280 F., and preferably from 1380 F. to 1300 F., at a rate of from about 2 to 10 F. per hour. It is specifically preferred to cool the material through a temperature range of about 1420 F. to 1280 F. at 10 F. per hour. The material or castings so treat-ed are then cooled at any desired rate to room temperature.

The following examples will illustrate the advantages of the present invention, and are not to be taken as limiting in any way.

Bar castings of white iron having a carbon content of 2.30% were placed in a furnace having the above specific atmosphere with a dew point of +45 F. (1.003% water vapor). The test bars were heated to a temperature of 1700i-20 F. at a rate of about 200 F. per hour, and held at this temperature for 20 hours. The bars were next cooled in the furnace to 1420 F. at the maximum furnace cooling rate. Then the test bars were cooled from 1420 F. to 1280 F. at 10 F. per hour, and then cooled to 800 F. at maximum furnace rate and removed from the furnace. It was found through measurements made with a cathetometer capable of being read directly to 0.05 mm. and estimated to 0.01 mm. that a length of bar which before annealing was about 10 inches long had grown or lengthened 0.89%.

When the above example was repeated in all respects except that the white iron had a carbon content of 2.81%, a growth of 1.23% was experienced.

When the above experiments were repeated, except that the dew point of the atmosphere was 40 F. (0.0188% H O), the 2.26% carbon containing castings had a growth of 1.57% and those containing 2.73% carbon lengthened 1.75%.

A number of bars were annealed in pot-type annealing furnaces, the bars being packed with gravel in sealed cast iron pots, heated to a temperature of 1600 F. for 36 hours, held at temperature for 50 hours, cooled to 1400 F. in 20 /2 hours, slow-cooled to 1300 F. in 16 hours, rapid cooled to 1000 F., and finally air-cooled to room temperature. Bars so treated had a growth of 0.89% for a carbon content of 2.33% and 1.35% growth for a carbon content of 2.90%

Shown in the table below are test results obtained with other percentages of carbon. In column I are the test results obtained by using the process of the present irivention. In column 11 are the results obtained with materials using a dew point of -40 F., while in column III the gravel pot method described above was used.

I II III Present Invention, Present Invention, Gravel Pot Annealing +45 F. Dew Point 40 F. Dew Point Carbon, Growth, Carbon, Growth, Carbon, G rowth, Percent Percent Percent Percent Percent Percent From the data presented above, it will at once be evident that the materials provided by the present invention are equivalent in all practical respects to those produced by the former excessively long and expensive gravel pot annealing method.

By this invention there is provided malleablized white iron which is much more readily made than by the old pot annealing method but with comparable results. The savings in time and heating are tremendous as is labor cost, cost for containers, which are replaced by longer lasting trays, and the cost of furnace maintenance. The present invention provides for more precise temperature control in the furnace and eliminates scaling which occurs in the pot annealing method when container seals fail. The final product produced is more uniform from lot to lot and the precise control of decarburization which is possible permits the adjustment of physical properties to be controlled thereby. As pointed'out above, the present invention permits the use of newer style furnaces without changing casting patterns since growth characteristics are essentially the same as in the pot annealing process.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The process of malleablizing white iron in a neutral to reducing atmosphere having added thereto water to produce a dew point of 0 F. to F. to produce black heart malleable iron having a growth of less than about 1% during the malleablizing process which comprises heating said white iron above the transformation temperature, holding until the cementite and alpha iron are converted to gamma iron and graphitic carbon and cool ing slowly through the transformation range to convert the gamma iron to alpha iron and graphite.

2. The process of malleablizing white iron in a neutral to. reducing atmosphere having added thereto water to produce a dew point of +20 F. to -|-50 F; to pro duce black heart malleable iron having a growth of less than about 1% during the malleablizing process which comprises heating said white iron above the transformation temperature, holding it until the cementite and alpha iron are converted to gamma iron and graphitic carbon and cooling slowly through the transformation range to convert the gamma iron to alpha iron and graphite.

3. The process of malleablizing white iron to produce black heart malleable iron having a growth of less than about 1% during the malleablizing process which comprises heating said iron in a controlled neutral to reducing atmosphere having water added thereto to produce a dew point of about 0 F. to +50 F. at a temperature of from about 1550 F. to 1800 F. for from about 2 to hours, cooling said iron quickly to a temperature of 1420 F. and then cooling to a temperature of about 1280 F. at a rate of about 2 to 10 F. per hour.

4. The process of malleablizing white iron to produce black heart malleable iron having a growth of less than about 1% during the malleablizing process which comprises heating said iron in a controlled neutral to reducing atmosphere having water added thereto to produce a dew point of about +20 F. to +50 F. at a temperature of from about 1600 F. to 1750 F. for from about 2 to 60 hours, cooling said iron quickly to a temperature of 1380 F. and then cooling to a temperature of about 1300" F. at a rate of about 2 to 10 F. per hour.

5. The process of malleablizing white iron to produce black heart malleable iron having a growth of less than about 1% during the malleablizing process which comprises heating said iron to a temperature of about 1700 F. for about 20 hours, cooling said iron quickly to a temperature of 1420 F. and then to 1280" F. at a rate of 10 F. per hour, in an atmosphere of a neutral to reducing character having water added thereto to produce a dew point of +45 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,915,091 Hayes et al. June 20, 1933 1,999,153 Gray Apr. 23, 1935 2,198,801 Boegehold Apr. 30, 1940 2,287,467 Carpenter et al. June 23, 1942 3,000,770 Wittmoser Sept. 19, 1961 FOREIGN PATENTS 917,571 France Sept. 16, 1946 571,245 Great Britain Aug. 14, 1945 613,514 Great Britain Nov. 30, 1948 

1. THE PROCESS OF MALLEABLIZING WHITE IRON IN A NEUTRAL TO REDUCING ATMOSPHERE HAVING ADDED THERETO WATER TO PRODUCE A DEW POINT OF 0*F. TO 50*F. TO PRODUCE BLACK HEART MALLEABLE IRON HAVING A GROWTH OF LESS THAN ABOUT 1% DURING THE MALLEABLIZING PROCESS WHICH COMPRISES HEATING SAID WHITE IRON ABOVE THE TRANSFORMATION TEMPERATURE, HOLDING UNTIL THE CEMENTITE AND ALPHA IRON ARE CONVERTED TO GAMMA IRON AND GRAPHITIC CARBON AND COOLING SLOWLY THROUGH THE TRANSFORMATION RANGE TO CONVERT THE GAMMA IRON TO ALPHA IRON AND GRAPHITE. 